There is well known automatic by-pass (Philips Company Catalogue PTV Professional Television Measuring Equipment, 1986-1987, p. 109) consisting of an electromagnetic relay with two contact switches where the common pole of the first contact switch is connected to the signal input, its operative pole being connected to the input of the processing unit and the load which, by its second end is connected to the ground of the input line, its non-activated pole being connected to the non-activated pole of the second contact switch, the operating pole of which is connected to the output of a processing unit and its common pole is an output of the automatic By-pass. The electromagnetic relay with its coil is connected to the positive and negative poles of the supply.
A disadvantage of the well-known automatic By-pass is that it reacts only when there is no supply from the processing unit, fully or partially, and does not consider any parameter change of the output signal, e.g. as a result of a deteriorating operation of failure of the output signal due to a defect. Another disadvantage is the possibility of unstable multiple reswitching, in case of an unstable or interrupted power supply which may jeopardize the signal transmission.